This invention relates to a noise barrier which combines high transmission loss and high sound-absorbing coefficient with excellent vibration proofing effect and which is directed to preventing noises and secondary noises issuing from wheeled vehicles travelling at high rates of speed and from machines productive of high solid sounds (sound waves generated from the surface of solid because of contact of two or more solid articles).
In recent years, high solid sounds generated by solid articles because of operations such as of cutting, deformation, friction and impact performed thereon and traffic noises generated by speeding vehicles on super highways and railroads have been degrading the life environment, posing a serious social problem. With a view to coping with this problem, there has been felt a strong need to develop a noise barrier capable of internally absorbing generated sounds and insulating sound waves and consequently preventing them from being propagated externally as well as measures for decreasing the absolute number of noise sources and means for lessening sounds in intensity. The sounds thus generated are propagated in conjunction with the vibrations which are generated by speeding vehicles and machines productive of solid sounds and the noises which are secondarily radiated because of said vibrations. Thus, the mere act of increasing the mass of the noise barrier or adding to the height of the noise barrier only results in an increase in the area of radiation of said secondary noises and cannot be expected to bring about the true effect of a noise barrier.
The present inventor formerly invented a noise barrier which has an extremely high transmission loss as compared with the conventional noise barrier and which comprises 1 a hollow cell type block composed of a hollow cell-shaped box and a sound-absorbing material filling said box, said hollow cell-shaped box consisting of a perforated sound-absorbing plate on the side confronting the source of sound and a sound-insulating plate opposed to said sound-absorbing plate and 2 a free-spacing plate disposed opposite said sound-insulating plate across a space parallel to said sound-insulating plate and connected with said plate by the medium of an elastic member so as to interpose a free air layer between said free-spacing plate and said sound-insulating plate.
Although the noise barrier of said construction has an extremely high transmission loss and is effective as a barrier to acoustic energy, it still suffers from a defect also common to conventional sound barriers in that it is not sufficiently effective as a barrier to vibratory energy.
The inventor continued a further study with a view to improving the basic structure of the noise barrier of said former invention. We have consequently acquired a knowledge that besides the sound energy in the ordinary sense of the word, the sound-producing energies which impinge upon the noise barrier can be divided into three types. The present invention has been accomplished on the basis of this knowledge. To be specific, the sound-producing energies fall under the following three types:
1. The acoustic energy which propagates in the air and penetrates through the noise barrier and, in the case of a noise barrier incorporating joints, also penetrates through said joints. PA1 2. The vibratory energy which propagates in the air to reach the noise barrier and produces vibration in the noise barrier. PA1 3. The vibratory energy which travels along the ground surface or through the elevated structure supporting the noise barrier and produces vibration in the noise barrier or its stationary auxiliary parts such as supporting columns.
The vibratory energy indicated in 2 and that indicated in 3 above will be collectively referred to hereinafter as "vibratory energy". In the case of a noise barrier against the sounds from speeding wheeled vehicles, for example, vibration-arresting rubber packing may be inserted to fill gaps in the assembly of the barrier structure for the purpose of enhancing the effect of intercepting the propagating sound. It has been experimentally established that this measure, if taken alone, does not prove very effective because it makes a limited contribution to the prevention of the vibratory sound generated by said three energies.
A major object of the present invention to provide a noise barrier having a larger transmission loss than a unitary noise barrier or even a noise barrier provided with a free spacing plate to form a free air layer and more specifically to provide a noise barrier which the effect of a free spacing plate is optimized by adoption of a vibration-repressing structure.
Another object of the present invention is to provide a noise barrier which is adapted to avoid generating secondary noise owing to the vibratory energy.
Still another object of the present invention is to provide a noise barrier which, on the basis of the experiment conducted on the effect of vibration from the supporting column serving to hold the noise barrier in position, is adapted so as to give the most effective attenuation of said vibration.